In the manufacture of articles from rubbery, viscoelastic and plastics working materials, sometimes called "plastic stock", it is common practice to filter the working materials. This may be done in a reclaiming process, to prepare re-claimed materials for use in a product; it may also be done, for example, in an extrusion apparatus, to make certain that no foreign matter will be introduced into an extrusion die. Strainers and filters for such working materials are known in many forms, perhaps an earliest one of which is the slide-plate filter shown in U.S. Pat. No. 642,814 issued to Cowen in 1900. Slide plate filters according to Cowen are still in use today. Other forms of flat plate filters which like Cowan's are intended to be used athwart an oncoming stream of plastic stock are shown in Garrahan's U.S. Pat. No. 1,195,576; Schneider et al. U.S. Pat. No. 2,607,954; Paquette 3,797,665 and Davis 2,878,512. These are by no means exhaustive of the prior art of such filters, but they do illustrate a property which is common to them, namely, that the breaker plate generally has a flat surface for receiving oncoming stock, and the area of filter surface which is presented across the stream of plastic stock to be filtered is essentially the same as a right-cross-section of that stream.
It is common practice to use filters of the type described in apparatus by which the plastic stock is pressurized; in some such apparatus the pressure applied may reach 10,000 pounds per square inch or more. Taylor et al U.S. Pat. No. 3,811,659 proposes arched strainer plates to prevent bulging and to contribute strength.
Pleated-screen filters have been used with relatively low-viscosity liquids, to increase the effective filter area presented to a flow of the material being filtered. Examples of pleated-screen filters are shown in U.S. Pat. Nos. of Rosaen 3,471,023; and Brown 3,747,772. As will be seen in those patents the pleated screens are supported by a breaker plate only at an array of points constituting a minor fraction of the total screen area, with the result that an attempt to use them to filter a plastic stock under sufficient pressure to force the stock through the screen would quickly crush the screen against the breaker plate, and effectively destroy its usefulness.